The present invention relates generally to a preparation for making a printed circuit board, and more particularly to a substrate of the printed circuit board.
The conventional method for making a printed circuit board involves the forming of through holes by a punch press or drilling machine prior to the formation of circuit pattern. The through holes are intended for use in connecting the components or circuits of adjoining layers of the printed circuit board by the bonding wires which are put through the through holes. The preformation of the through holes often results in the high rejection rate of the printed circuit board so made. For example, the formation of circuit pattern is adversely affected in view of the rugged surface which is formed on the substrate. In other word, it is technically difficult to planarize the rugged surface which is covered with a dry film photoresist. The technical problem persists even if a liquid photoresist is used in place of the dry film photoresist. In order to prevent the leak of the liquid photoresist via the through holes of the substrate, it is necessary to clog temporarily the through holes before the liquid photoresist is applied on the substrate. Upon completion of formation of circuit pattern on the substrate, the through holes are unclogged. The process of unclogging the through holes is rather time-consuming.
It is the primary objective of the present invention to provide a cost-effective process for forming through holes in a printed circuit board substrate.
It is another objective of the present invention to provide a through-hole forming process enabling a circuit pattern of high density to be formed on a printed circuit board substrate.
It is still another objective of the present invention to provide a through-hole forming process which can be carried out along with a process for forming a circuit pattern.
In keeping with the principle of the present invention, the foregoing objectives of the present invention are attained by the process involving the use of a substrate which comprises a resinous dielectric sheet and a conductive layer adhered to a first surface of the resinous dielectric sheet. A plurality of cavities are formed by laser on a second surface of the dielectric sheet such that the cavities penetrate the dielectric sheet, without penetrating the conductive layer. Thereafter, the substrate is coated on both surfaces thereof with a liquid photoresist of a predetermined thickness. The cavities are filled with the liquid photoresist. A plurality of small areas are formed by photolithography on the surface which is covered with the conductive layer. The small areas correspond in shape and location to the cavities. The conductive layer of the small areas is removed by etching before the cavities are stripped of the photoresist. The through holes are thus formed on the small areas defined by the cavities, which may be of any shape.
According to the present invention, the photoresist coating, the photolithography, and the etching are carried our along with forming a circuit pattern on the PLB substrate.